The present study clarified about the reproductive ecology of some littoral animals as follows.1. Interspecific breedings among three hexagramids were observed with underwater video camera and mtDNA and genomic DNA (calmosurin encording region) polymorphism analyses. The cross breedings did not occurred at random, but only two combination, between male Hexagrammos otakii and female H.octogrammus and between male H.agrammus and female H.octogrammus. All hybrids were female. DNA analyses detected F2 individuals, suggesting F1 hybrids are fertile.2. Primers of amplification of microsatellite DNA polymorphism were designed for paternity determination of Hypopthychus dybowskii. Males of this fish established a spawning territory around sargassum sea wedows. Five primers successfully amplified microsatellite DNA loci. Of them 4 sets of the primers showed high heterooccurrence frequency of 80% over. Relationship among density of fishes, density of spawning substratum and reproductive success is being investigated with the DNA polymorphism.3. A non-copulating sculpin, Hemilepidotus gilberti, produce much apyrene spermatozoa with plural nuclei during spermatogenesis. The function of apyrene sperm were investigated with fluorescent dye and microsatellite DNA polymorphism. It was showed that the sperm played as a broker toward sneaker's eusperm for sperm-competition.4. Primers of amplification of microsatellite DNA polymorphism were designed for cichlids in Lake Tanganyika, Telmatochromis temporalis and Julidochormis ornatus. The analyses resulted that the two species have a unique mating system, helpers and pirates sired with some of guarded eggs by pair.